Plug flow reactor model of the plasma chemical conversion of CO₂

Abstract

A 1D plug flow model suitable for describing the CO2 conversion into CO in microwave plasma reactors is proposed. The model is applied together with the Antwerp data set for the CO2 reaction kinetics to calculate parameter scans for a realistic experimental set up. The energy re-distribution pathways in the model calculations are analyzed. The analysis shows that despite the input power being initially deposited mainly into vibrational states the fast vibrational-translational (VT) transfer leads to dissociation of CO2 predominantly via the thermal quenching mechanism. Solutions with mitigated VT-losses can be obtained by increasing the specific input power–power per unit volume. In this regime the energy efficiency starts to be constrained by reverse processes.

Plasma Technology for CO2 Conversion: A Personal Perspective on Prospects and Gaps

“…It was indeed demonstrated, both experimentally (e.g., van Rooij et al, 2015;Bongers et al, 2017;den Harder et al, 2017;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020 and computationally (e.g., Berthelot and Bogaerts, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019), that in practice, the vibrational non-equilibrium is not the leading mechanism in (sub)atmospheric MW or GA CO 2 plasmas, and that the CO 2 conversion is thermal, although Pietanza et al (2020) suggested to act with caution on this assumption, as their model still showed clear deviations from equilibrium, even at temperatures of 3,500-5,500 K.…”

“…In the past decade, many papers, both experimental (e.g., Silva et al, 2014Silva et al, , 2017Klarenaar et al, 2017Klarenaar et al, , 2018Klarenaar et al, , 2019Stewig et al, 2020;Terraz et al, 2020;van den Bekerom et al, 2020) and computational (e.g., Bogaerts, 2014, 2015;Pietanza et al, 2015Pietanza et al, , 2017aPietanza et al, ,b, 2018aPietanza et al, ,b, 2020Bogaerts, 2016, 2018;Capitelli et al, 2017, Heijkers andGrofulović et al, 2018;Silva et al, 2018;Bogaerts, 2018, 2019;Kotov and Koelman, 2019;Terraz et al, 2020), focused on the role of the vibrational kinetics in plasmas, but mainly at low pressure conditions (few mbar up to a few hundred mbar). Most of these studies revealed the importance of the vibrational kinetics at specific conditions (low gas temperature, low pressure) and their contribution to a high energy efficiency.…”

“…In MW and GA plasmas, the electron temperature is still higher than the gas temperature (hence, they are not really thermal plasmas), but the vibrational and gas temperature are often close to each other (i.e., VT equilibrium), so they can also be considered as quasi-thermal plasmas. For this reason, the CO 2 conversion proceeds mainly by thermal reactions in MW and GA plasmas at practical operating conditions (van Rooij et al, 2015;Berthelot and Bogaerts, 2017;Bongers et al, 2017;den Harder et al, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020.…”

“…In previous modeling (e.g., Berthelot and Bogaerts, 2017;Heijkers and Bogaerts, 2017;Sun et al, 2017;Kotov and Koelman, 2019) and experiments (e.g., Bongers et al, 2017), suggestions were provided on how the vibration-induced pathway could be exploited to optimize the energy efficiency at practical MW and GA conditions. For instance, Bongers et al (2017) studied CO 2 dissociation in a vortex-stabilized MW plasma reactor in a wide range of conditions regarding MW field, residence time, quenching, and vortex configuration.…”

“…The above suggestions to improve the energy efficiency by promoting the vibrational dissociation pathway are not straightforward to realize in practice. Indeed, recent insights based on both modeling and experiments have revealed that the CO 2 conversion in (sub)atmospheric pressure MW and GA plasmas proceeds by thermal reactions (van Rooij et al, 2015;Berthelot and Bogaerts, 2017;Bongers et al, 2017;den Harder et al, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020, and this can also give rise to quite high energy efficiencies (up to 40-50%). As explained above, the rate coefficients of the thermal reactions between various plasma species rise upon higher gas temperature, thus enhancing the conversion.…”